Hydroformylation is a well-known process in which an olefin is reacted with carbon monoxide and hydrogen in the presence of a catalyst to form aldehydes and alcohols containing one carbon atom more than the feed olefin. This process has been operated commercially for many years and there have been two principal technology families used. One is known as the low or medium pressure oxo process family and which generally involves the use as catalyst of an organometallic complex of rhodium with organophosphorous ligands for providing the necessary stability at the lower pressures and operates at pressures from 10 to 50 Bar. The second process family is known as the high or medium-pressure process family and generally involves the use of a cobalt or rhodium based catalyst and typically operates at pressures from 50 to 350 Bar. Generally the low pressure processes are used for the hydroformylation of unbranched and terminal, primarily lower olefins such as ethylene, propylene and n-butenes, but also including n-hexene-1, n-octene-1 and mixtures of higher carbon number terminal olefins produced by the Fischer-Tropsch process. The high or medium pressure processes are primarily used for the hydroformylation of linear internal and branched higher olefins such as those containing 5 or more carbon atoms. This process is widely used to produce what are known as “higher alcohols” or aldehydes or acids which are in the C6 to C15 range particularly the C9 to C13 range. Such materials are typically used in the production of plasticiser or lubricant esters such as the esters of phthalic acid and anhydride, esters of cyclohexane dicarboxylic acids, esters of adipic or trimellitic acid, esters of the various isomers of pyromellitic acid, and polyol esters, but also in surfactant derivatives like ethoxylates, sulfates, or ethoxysulfates.
Hydroformylation is typically performed in large volume reactors which may be continuous or batch reactors. The present invention is concerned with hydroformylation that is performed in a series of at least two reactors.
The present invention is concerned with improving hydroformylation reactions operating continuously and at high or medium pressures and generally using a cobalt based catalyst although other catalyst systems may be used.
As with most large scale industrial chemical processes improvements in the efficiency of the use of raw materials, optimisation of the recycle of unreacted materials and the optimisation of reaction conditions, material balance and other variables are most important. Improvements which can result in a few percentage point increases in conversion, output and efficiency are extremely significant improvements.
High and medium pressure hydroformylation (sometimes known as OXO) reactions involve the reaction of liquid materials with normally gaseous materials which are at least partly dissolved in the liquid during reaction due to the high pressure conditions, and gaseous materials may also be entrained as droplets or bubbles in the liquid phase. Unreacted materials are vented off after the reaction and the present invention is concerned with optimising the reuse of such off-gases in the reaction system. The invention is particularly concerned with the optimisation of gas utilisation by the use of a combination of fresh gaseous feeds and the recycle gases to optimise reaction conditions and thus the conversion and yield of the hydroformylation reaction.
The starting liquids that are involved in high pressure hydroformylation comprise olefins which may be mixtures of olefins such as those obtained from olefin oligomerisation units. For example the olefins may be mixtures of C5 to C12 olefins obtained by the phosphoric acid catalysed oligomerisation of C3 and C4 olefins and mixtures thereof, and where olefin mixtures are used, they may have been fractionated to obtain relatively narrow boiling cut mixtures of mostly the appropriate carbon number for the production of aldehydes and alcohols with the desired carbon number. Alternatively the olefins may be obtained by other oligomerisation techniques such as for example the dimerisation of butene using a nickel oxide catalyst, like the Octol® process, or an oligomerisation process for ethylene, propylene and/or butenes using a nickel salt and involving di-alkyl aluminum halides, like the range of Dimersol® processes, or a zeolite catalyst. The olefins may also be obtained from ethylene growth processes, in which case they are often called linear alpha olefins or normal alpha olefins, or they can be mixtures obtained from the Fischer Tropsch process, which primarily contain terminal olefins but which may show some side branches along their longest alkyl chain.
The gases that are involved in high and medium pressure hydroformylation include carbon monoxide and hydrogen, frequently supplied in a mixture that is known as synthesis gas or “syngas”. Syngas can be obtained through the use of partial oxidation technology (POX), or steam reforming (SR), or a combination thereof that is often referred to as autothermal reforming (ATR). It can be generated from almost every carbon containing source material, including methane, natural gas, ethane, petroleum condensates like propane and/or butane, naphtha or other light boiling hydrocarbon liquids, gasoline or distillate-like petroleum liquids, but also including heavier oils and byproducts from various processes including hydroformylation, and even from coal and other solid materials like biomass and waste plastics. On liquid feeds, a steam reformer may involve a pre-reformer to convert part of the feed to methane before entering the actual reformer reaction. The gaseous feed streams can also contain inert components such as nitrogen, helium, argon, methane and carbon dioxide which, although mostly inert, are significant in that they have a dilution effect and can help to control reaction temperature. The drawback with these inerts is that their concentrations may change over time, which changes their effects in the hydroformylation step. Nitrogen, helium and argon can come in as impurities with the oxygen supply, or when air or enriched air is used as feed for syngas generation. Also natural gas or methane feeds can contain such inerts. Methane can be left over from incomplete conversion during the syngas generation, or from a methanation reactor that may be included in the flow scheme. As source levels can change, and operating conditions can change, so can the levels of these inerts. Carbon dioxide levels are controlled by reaction conditions in the syngas generation or by absorber and regenerator efficiency if an acid gas absorber/regenerator system is included downstream thereof. Carbon dioxide levels in the fresh syngas are therefore prone to change too.
The gases that are involved in the present invention are fresh gases and the recycle of unreacted gases. An important aspect of the present invention is the use of recycle gases to obtain the appropriate balance of gases in the reactors. Compression of gases, particularly when these need to be brought up to medium and high pressure levels, is energy intensive, and especially inefficient in energy use when the gases comprise significant amounts of hydrogen. Separating unreacted gases from the hydroformylation reaction product, and the partial recycle of this offgas to the reactor, is a well-known way to improve the overall utilisation of the gas feeds to the hydroformylation reactors. It is known that with this gas recycle, due to the presence of inerts in the fresh gases, inerts will build up in the system and reduce the partial pressures of the reactive components like hydrogen and carbon monoxide. Purging a portion of the offgas mixture is the typical solution to avoid excessive buildup of inerts.
Hydroformylation reactions may be continuous or batch reactions and the present invention is concerned with continuous reactions. The reactions with which the invention is concerned take place in a series of two or more reactors and in a preferred embodiment the reactions take place in a series of reactors involving gas lift reactors as lead or front end reactors, more preferably involving loop reactors. The series of reactors may be made up of separate distinct sections within one or a few reaction vessels. Alternatively, one reactor in the series can in itself be made up of different volumes set up in series or in parallel.
The main hydroformylation reaction is the reaction of an olefin with carbon monoxide and hydrogen to produce an aldehyde,Olefin+CO+H2→Aldehyde
This reaction consumes equimolar amounts of CO and H2. Its rate of reaction is proportional to the ratio of hydrogen-to-CO. When the fresh feed syngas does not contain CO and hydrogen in equal molar amounts, this H2/CO ratio is bound to change as the reaction progresses through the hydroformylation reactor zone. Also, there are a number of competing and consecutive gas consuming reactions in which the CO and H2 is not necessarily consumed in an equimolar ratio, for example:Olefin+H2→ParaffinOlefin+CO+H2O→AcidAldehyde+H2→AlcoholAldehyde+CO+H2→Formate esterAldehydes can condense with alcohols to form a hemi-acetal, R1-CHOH—O—R2, which is not very stable and splits off water to form an unsaturated ether. This again can undergo gas consuming reactions:Unsaturated ether+H2→di-alkyl etherUnsaturated ether+CO+H2→ether aldehyde
All these other gas consuming reactions also affect the quantity of CO and hydrogen present throughout each reactor and throughout the series of reactors. Also the rates of these reactions are dependent on conditions and concentrations at each local spot in the reaction zone which are not necessarily constant. The presence of CO and hydrogen is therefore not constant throughout the reaction zone in absolute terms, and even less in relative terms particularly the case when two or more reactors in series are used.
There remains therefore a need to control the concentration and ratio of CO and hydrogen in the second and any subsequent reactors in the ranges desired for that part of the reaction zone. The present invention provides such a control.
In an industrial hydroformylation plant that is producing alcohols, at least part of the product of hydroformylation which consists primarily of aldehydes, or of mixtures of alcohols, aldehydes and formate esters, potentially together with various other compounds, is hydrogenated to convert the aldehydes and formate esters to alcohols and to reduce the level of the impurities. On certain catalysts, these formate esters can, amongst others, split into an alcohol and carbon monoxide, which in the presence of water, can undergo the water-gas-shift reaction and produce hydrogen and carbon dioxide. Alternatively, the formate ester can hydrolyse with water to form the alcohol and formic acid, which then can decompose into carbon monoxide and hydrogen. This formed hydrogen is then available to participate in the hydrogenation reaction. Overall, the hydrogenation reaction is typically operated with a stoichiometric excess of hydrogen, and can therefore result in a stream of unreacted hydrogen. In one aspect, the present invention is therefore also concerned with the use of this stream, to provide the optimum gas balance in the second and subsequent reactors in hydroformylation reactions involving a series of two or more reactors. As with the other streams this hydrogen off gas stream may contain inerts such as nitrogen and methane. It can also contain carbon dioxide as explained earlier. Typically it does however not contain significant amounts of carbon monoxide, because this is against the equilibrium of the water-gas-shift reaction (H2O+CO<--->CO2+H2) that is approached over many catalysts used in the hydrogenation step. Carbon monoxide in the offgas from hydrogenation is typically below 1 mol %, preferably not more than 0.5 mole %, more preferably not more than 0.4 mole %.
It is known, from WO 94/29018, to conduct hydroformylation reactions in a series of reactors and it is also known from WO 94/29018 to feed fresh olefin and fresh synthesis gas to the first and second reactors. This split feed has been found to be beneficial because the first reactor, when it is a gas-lift reactor or preferably a loop reactor, in which the internal fluid circulation and mixing is driven by a density difference provided by having more gas in the upward moving part of the reactor, as compared to the downward moving part, can become unstable as the olefin feed rate, particularly also combined with the additional amount of gas required for the reaction, is increased above a certain threshold. The maximum amount of olefin that can be fed to the lead reactor depends upon many factors, but also on the reactivity of the particular olefin that is being processed. More stable conditions can be realised if the fresh olefin feed is divided between the first and second reactors. It has also been found that if the optimum amount of syngas is fed to the first reactor there can be hydrodynamic instabilities due to the large volumes of gas present. These instabilities can also lead to fluctuations in the temperature in the reactor. This can lead to unique steady state conditions that are periodic, also known as limit cycles. Further increases may enlarge the limit cycle range, and ultimately lead to more severe conditions of a multiplicity, usually a runaway or, at startup, an inability to start the reactor. This situation may be managed by adjusting temperature and catalyst concentration, but it is preferred that a part of the fresh syngas also be fed to the second reactor to alleviate the risk and consequences of the instabilities. Another way to cope with possible instabilities in the lead gas-lift reactor is to provide two lead reactors in parallel, both fed independently and both feeding their product into a third reactor which is in the second position. This method may still be combined with the split feed of olefin and/or fresh syngas to the reactor in second position and in the second reactor for the purposes of this invention. U.S. Pat. No. 4,320,237 also employs a series of reactors.
U.S. Pat. No. 3,378,590 hydroformylates olefins and synthesis gas using a single hydroformylation reactor and cobalt catalysis. Gases that are unreacted during the hydroformylation reaction are recycled to the hydroformylation reactor. In U.S. Pat. No. 3,378,590 the product of hydroformylation containing cobalt carbonyls passes to a hydrogenation zone where the aldehydes are hydrogenated to alcohols and the metal carbonyls are decomposed to the metal and carbon monoxide. The mixture of unreacted hydrogen and the carbon monoxide can be recycled to the hydroformylation reactor.
In U.S. Pat. No. 4,049,725 a mixture of hydrogen and carbon monoxide is used for the hydrogenation of aldehydes and the partially hydrogen depleted waste stream that is obtained after this hydrogenation is used in the hydroformylation reaction.
The ratio of hydrogen to carbon monoxide is an important criterion for the performance of the hydroformylation reaction. As stated by Falbe on page 17 of his book “New Syntheses with Carbon Monoxide”, the reaction rate is directly proportional to that ratio. He also states that for each temperature and cobalt concentration, there must be a minimum partial pressure of carbon monoxide present to keep the cobaltcarbonyl catalyst stable, so that plating out of the cobalt as cobalt clusters or as metal does not occur. As a consequence, within these requirements of a minimum carbon monoxide presence, the higher the hydrogen concentration the better.
Hydroformylation reactions typically do not convert all the reactive materials fed to the reactor and so the product of hydroformylation contains unreacted materials particularly carbon monoxide and hydrogen. The traditionally applied excess of carbon monoxide assures the stability of the catalyst even after most of the reaction has taken place. A hydrogen excess is typically desirable to keep the reaction rate up also in the back end of the reactor section.
The products of hydroformylation, predominantly aldehydes and alcohols, but also some formate esters and possibly including heavy materials, are typically fed at least partially, as such or optionally after distillation, to a hydrogenation reactor. This is usually done after catalyst removal, and the products are then hydrogenated to produce alcohols. There is generally an excess of hydrogen in the hydrogenation reactor, which is left over as a waste stream. Optionally, a part of the aldehydes from the hydroformylation reaction may be separated off for use as such or for being converted into carboxylic acids.
There are therefore many gas streams in the commercial manufacture of alcohols and the present invention is concerned with the optimisation of gas utilisation throughout the hydroformylation reaction and also throughout a process involving both hydroformylation and hydrogenation.
The present invention is illustrated by reference to FIG. 1 which is a schematic diagram illustrating a hydroformylation facility employing a gas cycle according to the present invention.